1. Technical Field
The inventive concept relates to a wavelength tunable light source apparatus, and more particularly, to a wavelength swept source apparatus for sequentially and rapidly emitting a plurality of light rays having different frequencies.
2. Description of Related Art
A wavelength swept source apparatus sequentially emits a plurality of rays having different frequencies. The wavelength swept source apparatus is used in various fields such as the fields of a real-time optical coherent tomography (OCT) instrument for monitoring an industrial process, a medical three-dimensional OCT instrument, a light source for optical communication, a semiconductor analyzer, a polymer analysis spectrometer, and a light source for an environmental gas monitoring apparatus.
The wavelength swept source apparatus may be construed as one type of a wavelength tunable light source apparatus, in a broad sense. The wavelength swept source apparatus may be classified into a bulk optics type, a fiber optics type, and a semiconductor integrated optical element type, according to a type of a resonator. According to the bulk optics type apparatus, a wavelength of light may be changed within an optical gain bandwidth of a light source, a resonator may be easily implemented, and an optical loss is small. However, the bulk optics type apparatus needs precise optical alignment and has a large volume. Moreover, the bulk optics type apparatus has low reliability due to its strong mechanical vibration.
According to the fiber optics type apparatus, the wavelength swept source apparatus may be implemented in various configurations and forms according to linear or nonlinear characteristics of an optical fiber. However, the fiber optics type apparatus has a large volume and includes a large number of components, accordingly, has high unit cost of the apparatus. Furthermore, when a frequency of light outputted from the fiber optics type apparatus varies rapidly, intensity of the output light is reduced and linewidth of the output light is broadened.
The semiconductor-integrated optical element type apparatus has a small volume and high reliability, and may be manufactured cost-effectively. However, according to the semiconductor-integrated optical element type apparatus, an oscillation mode may be discontinuously switched while a wavelength is tuned, i.e., a mode hopping phenomenon may occur. Moreover, according to the semiconductor integrated optical element type apparatus, a method to change a frequency is complicated.
In the past, each of the above-mentioned implementation types had distinctly different performance, advantages, and disadvantages. Thus, each of the implementation types was applied to different fields. However, according to the development of implementation technology and the improvement of apparatus performance, the limitations of each of the implementation types have been recently overcome. Accordingly, the fields to which each of the implementation types are applied overlap with each other.
The OCT or automatic optical inspection (AOI) system using the wavelength swept source apparatus needs to provide a high-definition image in real time. In these systems, an apparatus for rapidly changing a frequency of output light should be used. A tuning rate of about 1 MHz or higher is required so that these systems provide at least five sheets of a high-definition three-dimensional image per second. However, the typical implementation types have limitations in terms of a frequency changing speed of output light and operation stability. Thus, the typical implementation types have a tuning rate of several tens to several hundreds of kHz.